Flocculants are reagents which are added to suspensions of solids to cause the solids to agglomerate (or floc) and settle or dewater more efficiently. A highly effective type of flocculant is the polymeric type which may be prepared in numerous variations. Polymeric water-soluble flocculants may be nonionic, anionic, cationic, or amphoteric. The particular type of flocculant to be used in a given application may depend upon the nature of the surface of the suspended solids and other factors such as pH.
Acrylamide monomer, a nonionic, is a basic building block for water soluble polymers because of its price and availability. It may be homopolymerized to obtain nonionic polymers. It is frequently copolymerized with one or more monomers containing primary, secondary, tertiary, or quaternary amine groups to obtain cationic flocculants. Similarly, it may be copolymerized with acrylic acid, 2-acrylamido-2-methyl propyl sulfonic acid and other monomers containing acid groups to obtain anionic flocculants.
Water soluble copolymers containing sulfonate groups are known in the art to be effective flocculants. U.S. Pat. No. 3,692,673 to Hoke teaches the use of polymers of 2-acrylamido-2-methyl propyl sulfonic acid and their salts as flocculants particularly for use in clarification of surface water and municipal sewage. While Hoke uses a low solids suspension of kaolin (200 ppm) to approximate surface water turbidity, the present invention is different in that Hoke works with a 200 ppm suspension of kaolin while the present invention is concerned with 10% or more mineral slurries (100,000 ppm or more). The aqueous mineral suspensions containing 10% or more solids treated in the present invention exhibit much higher viscosities of at least 100 cps or greater than the 200 ppm suspension of Hoke. Such high viscosities tend to inhibit thorough and even distribution of the flocculant among the mineral particles, said thorough and even distribution being necessary to achieve flocculation. Moreover, the objective of the work reported in Hoke is supernate clarity, i.e. improved solids removal and/or improved settling rate, whereas the present invention involves, for example thickening and/or dewatering. Thickening or dewatering, i.e. the creation of a compact filter cake containing as little water as possible, places different and more stringent requirements on a flocculant than the separation of solids from water without regard to the condition of the separated solids.
In U.S. Pat. No. 3,617,572 Monagle teaches the use of copolymers of acrylamide and an alkali metal salt of vinyl sulfonic acid as effective flocculants for improved settling of clays, including kaolin, which are present as impurities in concentrated salt solutions. Again, Monagle is concerned with improved solids capture, i.e. clarity or settling rate, which is distinct from the objective of the instant invention. It is recognized in the art that vinyl sulfonate polymerizes very, very sluggishly compared to acrylamide. As a result, copolymers of vinyl sulfonic acid and acrylamide are very likely block-like. Conversely, 2-acrylamido-2-methyl propyl sulfonic acid (AMPS) and its salts polymerize at a rate only slightly less than that of acrylamide; acrylamide/AMPS copolymers are approximately random copolymers. Due to the significant difference in intra-molecular structure, any performance results of VSA/AM copolymers would not anticipate or predict the results to be obtained with AMPS/AM copolymers.
Finally, in U.S. Pat. No. 4,372,653, Halverson teaches the use of copolymers of 99-65 mole percent acrylamide and 1-35 mole percent 2-acryl-amido-2-methyl propyl sulfonic acid and having Brookfield viscosities of at least 2.0 cps at 0.08% concentration in one normal NaCl as effective flocculants especially useful for treating phosphate slimes. Halverson also describes the use of AMPS/AM copolymers as flocculants for human wastes, and for processing streams encountered in the copper, uranium, potash and coal industries. It is noteworthy that despite the several flocculation applications described for other mining processes, Halverson does not consider dewatering, particularly of Kaolinitic clays. This is not surprising, since one skilled in the art would not normally consider the use of a flocculant where redispersion is essential as with the dewatering of Kaolinic clays. Halverson describes the use of AMPS/AM polyumers as coagulant aids, in conjunction with alum, for color removal. All the performance data reported by Halverson pertains to improved settling rate or reduction of the volume of the underflow.
The reader may also be interested in Engelhardt et al U.S. Pat. No. 4,357,245, which describes a drilling mud containing 2-acrylamido-2-methyl-propane-3-sulfonic acid copolymerized with acrylamide, and the prior art reviewed therein. See also Engelhardt et al U.S. Pat. No. 4,309,523, also describing a drilling mud, and Lim et al U.S. Pat. No. 4,077,930 and 4,147,681 which describe self-inverting emulsions of similar polymers. Polymers similar to the ones I employ have also been used by Adams et al in U.S. Pat. No. Re. 29,595, Randin in U.S. Pat. No. 4,296,016, and Doggett et al U.S. Pat. No. 4,136,078 for various purposes.
Despite these and many other teachings, no one has yet developed a flocculant which will effectively flocculate aqueous pigment slurries such as Kaolinitic clays for dewatering or thickening and subsequently permit effective redispersion of the thickened or dewatered slurry to obtain a high solids, e.g. 60-70% or more solids, low viscosity product. This is not too surprising when one considers that flocculation and dispersion of solids in a slurry are totally opposite phenomena. A flocculant causes the particles to agglomerate into larger clumps which settle or dewater more rapidly. A dispersant functions by causing the individual particles to repel--the opposite mechanism.
Approximately 5 to 6 million tons of kaolin clay per year are mined, processed and shipped from the Southeast United States. The clay is usually mined by the open pit method. It is then slurried in water at about 20-30% solids, dispersed with various combinations of inorganic and organic dispersants to maintain a low viscosity, and processed. Processing includes removal of iron and other magnetic impurities, removal of sand and silt, bleaching for whiteness and brightness, and, finally, dewatering, usually on rotary vacuum filters, to obtain a filter cake which is 50-60% solids. Part of the filter cake is spray dried, packaged and sold as solid dry clay. A portion of the filter cake is spray dried and blended back with the remaining filter cake to obtain a 70% solids aqueous slurry. Additional dispersants may be added to the 70% slurry as needed to obtain a final Brookfield viscosity of less than 1000 cps (preferably less than 500 cps). This low viscosity 70% slurry is then shipped in bulk to the customer.
While many high molecular weight organic polymers--cationic, anionic and nonionic--have been shown to be capable of flocculating kaolin clay in a satisfactory manner, non has been able to do so without having significant deleterious effects in the blending back or redispersing of the 70% slurry. The presence of the flocculant in the slurry normally will work against the desired effect of the dispersant, typically a low molecular weight polyacrylic acid, to disperse the solids and lower the viscosity. The necessary low viscosity for the final product has not been achieved to date.